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This invention relates to receiving a radiating input wavefield that propagates in a medium and transmitting a radiating output wavefield in a medium. Example applications in the field of the invention include general optical devices, radar systems, ultrasonic medical imaging systems, ultrasonic industrial inspection, seismic prospecting, and sonar. High power applications, where waves modifiy materials, are included in this field as well as low power applications, where waves pass through a medium without modifying the materials.
A wavefield operates in space and time. Typical use of this term is in connection with waves in three dimensional space where a quantity such as magnitude of an electric field varies over both space and time. Describing such a wavefield involves three dimensions of space as well as time. In this document the term “wavefield” is used to indicate one or more wavefields that operate in at least two dimensions of space. Where a wavefield has a spatial form that can be represented by a wavefront that is a surface of points of equal phase, that wavefield is spatially coherent. Waves naturally propagate from a point source in a clear medium as coherent wavefields that can be represented by spherical wavefronts.
A restricted wave is a wave on a wire or in a waveguide where the electric field varies along its transmission path as a function of time. Describing this restricted wave involves one dimension of space and one of time, but often the space dimension can be ignored. Such a restricted wave is here called a signal in a channel. A wavefield may also be restricted to a planar surface such that only two dimensions of space and one of time can fully describe its operation.
A transformer is a standard device in electrical and electronic systems. Electrical power systems use transformers to modify voltage for power distribution where there is an input voltage and an output voltage. Another use of the transformer is to match impedances such that maximum power is transferred. Such transformers are passive devices where output power is no more than input power. Such standard transformer applications relate to electrical variations as a function of time only. The most common transformer produces an output waveform in response to an input waveform in accordance with the ratio of turns of wire in the secondary winding to turns of wire in the primary winding. This rule is applied for an arbitrary input waveform that is applied to the input point. For a linear transformer device the principle of superposition applies such that two waveforms that are combined at the input will both be subjected to the same transformation process in parallel. If the input waveforms are simultaneous, then the parallel transformation processes will be simultaneous.
Transformers operate in reverse. A discussion about operation in one direction implies a corresponding operation in the reverse. This is generally assumed for all devices discussed in this document, with appropriate modification of device details where necessary. For example, transmit operations of radars generally utilize much of the same equipment as receive operations, but where transmit and receive functions are involved there is a separation of these functions.
Optical devices have a general purpose of modifying optical waves. Counterparts are known in prior arts of radar, sonar, and ultrasound. Such a purpose is accomplished where input wavefields are continuously sampled over a surface. Mirrors, lenses, prisms, gratings, screens, and apertures in screens are examples. All of these devices carry out a transformation of an input wavefield. For an input wavefield that is a composite of component wavefields, transformations take place for all component waves as parallel processes. Except for some high power wavefield cases, these devices usually operate as linear devices so the principle of superposition applies such that waves from multiple sources that simultaneously arrive at an input surface undergo transformations that are simultaneous, without significant mutual effects. Lenses are most commonly associated with optical waves, but ultrasonic lenses and radar lenses accomplish similar functions through their operation as refractive devices.
Signal processing is a field of electronic engineering with counterparts in physics, including optics. The electronic category has expanded greatly with the availability of special digital signal processing (DSP) integrated circuits. The general field still includes many analog devices such as amplifiers, filters, mixers. Switches are more for the purpose of modifying a configuration rather than changing the form of a signal. Delay devices and phase shifters also operate without modifying form of the signal. The general field is mostly organized much like operation of the transformer. A network is typically defined that has a transfer function that defines the output response at an output point to an input response at an input point. Much of this field is based on the capability of transistors to amplify signals. Amplifiers are a form of active device. An active device causes active signal modification where an output signal carries more power than the corresponding input signal.
Great strides have recently been made in electronic signal processing, but in spite of this the field of optical signal processing continues to have major advantages in processing speed since a propagating wavefield represents many processes that are painstakingly slow in electronic signal processing. Many electronic channels are needed to do things that are carried out by a single focused wavefield. For example, an optical processor that is a simple lens is all that is required to cause such a focused wavefield that delays and sums a continuous wave that is equivalent to a very large number of channels. However, a lens is strictly a passive device without the flexibility of electronic devices to include active devices that increase power in a signal. A lens is also fixed after it is constructed where electronic processes can be readily varied. Lenses have generally been used for direct processing of wavefields where the form of energy is essentially the same throughout the process. For example, light waves vary only in speed as they enter the lens, travel through lens material, and exit the lens. Lenses have also been used for indirect wavefield processing where form of the wave energy is modified prior to wavefield processing. Acoustic holography is an example of this latter mode, where form of wave energy is interchanged from acoustic waves to light waves.
A previous patent U.S. Pat. No. 6,524,248 B1 Bullis (Febuary 2003) disclosed an aberration correction system that included a forward propagation device that carried out a wavefield transformation process where an input wavefield is transformed into an output wavefield using a one-to-one mapping process. The mapping was done with many parallel channels that conveyed signals from an input surface to an output surface. This caused an output wavefield that included component wavefields, one of which was a collimated wave and others were spherical waves. Differences in propagation of such components enabled separation of distortion signals arising at multiple source points from an intended signal arising at an intended focus point. The multiple distortion signals were separated and evaluated to determine correction signals that operate in parallel. This disclosure was written with the purpose specifying an aberration correction device and did not develop the general purpose nature of this transformation device.
U.S. Pat. No. 5,428,999 Fink (July 1995) disclosed a time reversal mirror that carried out a reverse propagation process in an inhomogeneous medium for the purpose of compensating for wave speed irregularities. Received signals were stored and retransmitted from the same devices that were used to receive the signals.
While much of the field of electronic signal processing addresses the topic of point to point transforming of signal waveforms, the task of receiving wavefield signals occasionally arises. Such operations are sometimes called spatial signal processing. For reception of input wavefield signals, the wavefields are typically converted to time domain signals at sample points in space that are on the surface of an array, and then conveyed in electronic channels to an electronic summing point. The channels are configured such that, for an input wavefield that originates at a source point in space, a signal at the summing is a response to a signal at that source point. The process of selecting different source points is accomplished by adjusting time delays in the channels. Sequential operations with different time delay sets allows sequential sensing of wavefield components from multiple source points. This type of signal processing is sequential beamforming. Parallel operations are possible where multiple summing points are arranged, where each summing point has its own set of channels from the sample points. Each of the multiple summing points produces a signal and the signals are responses to respective wavefield source points. Such electronic processing carries out a limited type of lens function, with more flexibility in varying configurations. Where input and output surfaces are shaped glass, there is obviously no opportunity for variation after fabrication. Where input and output surfaces are transducer arrays, the shapes are also difficult to vary but the electronic processing flexibility helps to restore some flexibility.
A device that operates similarly to the sequential beamforming process is called a fiber optic lens, as disclosed in U.S. Pat. No. 4,650,279 Magee (March 1987). The equivalent of the summing point is a fixed focus point that is on the lens axis. This special form of lens utilizes optical fibers to connect input points on an input aperture to output points on output aperture, and is thus limited to waves that can be sampled by and conveyed in an optical fiber. In this invention, input wavefronts representing a wave from a fixed source point that is at a fixed direction are made to converge to a receiving point. In doing so, input wavefronts are transformed into curved wavefronts that represent a focused wave that concentrates energy at the receiving point. While this device utilizes a wavefront transformation effect, its purpose is to reduce the wavefield to a signal at a single point. The specification directs that the point where energy is concentrated is set in fabrication of the device. In a first embodiment, the source point is also set in fabrication.
However, U.S. Pat. No. 4,650,279 Magee (March 1987) also provides for adjustments that can be made immediately prior to receiving operation. These adjustments are for changing effective pointing angle of the device to sense a source point in a different direction. After adjustment the spatial form of the input wavefield and the spatial form of the output wavefield are fully specified. The only unknown is the time function of the wavefield signal.
Because U.S. Pat. No. 4,650,279 Magee (March 1987) is designed for a limited purpose telescope application, it specifies contrary to concepts of operating on wavefronts having arbitrary spatial form and of parallel operation for multiple input wavefields. It also specifies contrary to the concept of simultaneously focusing wavefields from multiple points in space onto a focal plane. These concepts are among known functions of optical lenses that are beyond reach of this limited invention. This is evidenced by specification of a single receiving point on the device axis and by a pre-set source point location.
The fiber optic lens of U.S. Pat. No. 4,650,279 Magee (March 1987) is also limited as a passive device. Even though active methods are an obvious part of devices to change the length of the fibers, the signal in the fibers neither increases in power nor significantly changes in form as it passes through the extended fiber.
Radar lenses are also known but these have similarly limited purpose. Barton provides a survey discussion of such radar lenses in Skolnik (editor), Radar Handbook, McGraw-Hill, Inc., 1970, pp 10-19-10-24. While radar lenses operate on the basis of refraction effects, configurations are discussed in this reference where a lens material is compartmented by an arrangement of metal surfaces or a lens effect is arranged by a waveguide effect of metal surfaces. Such compartments operate with wavefield signals in the upper range of radar frequencies where waveguides are effective. Wavefields arrive at an input surface where they enter the compartments that convey the signals to an output surface. The compartments confine a portion of the input wave energy and deliver it to an output point. Barton discusses various arrangements where, like the above discussed fiber optic lens, the radar lens is steered in direction such that a single focus position on the lens axis is maintained. An alternative is a method of steering that is a sequential motion of a focus point over a focal surface. These methods are thus specified for wavefields of pre-determined spatial form. They also preclude parallel operation for waves from multiple sources. Because it is natural for radar systems to rely on transmit beams and receive beams, it is usual practice to receive only for points illuminated, and thus multiple simultaneous receive channels are uncommon. Thus discussions of radar lenses naturally preclude simultaneous focusing of wavefields from multiple sources on a focal surface. As with the fiber optic lens, radar lens devices are fundamentally passive devices.
In radar there are some devices called lenses that are actually electronic beamforming devices. Such misnamed lenses utilize channels to convey sampled signals from an input wavefield directly to summing points by electronic transmission devices, without use of a secondary, radiating output wavefield. Knittel describes such a device in Brookner (editor), Radar Technology, Artech House, Inc., 1977. pp 300-301.
Transducers are devices that change the form of energy through a conversion process. These include such devices as piezoelectric sensors that change pressure waves into electrical signals. Arrays of transducers are known for the purpose of spatially sampling a wave over an aperture surface. Typically, electronic signal processing carries out the necessary functions to convert the wave samples into an electronic signal. Definitions become less clear where waves are electromagnetic processes in free space and signals are electromagnetic processes in somewhat confined space, as in a waveguide. However, a waveguide opening is a form of transducer that simply channels wave energy from free space into the waveguide. Reciprocal processes are understood to be similar. The fiber optic lens as disclosed in U.S. Pat. No. 4,650,279 Magee (March 1987) involves light entering the optical fibers and being then confined to the fibers similar to the way lower frequency electromagnetic waves entering waveguides. In the same sense, entry and exit from an optical fiber can be considered as a transducer operation.
Arrays of transducers are widely used. Medical imaging is an area that utilizes ultrasonic transducer arrays. Most of these involve transducers that are arranged along a line that is either a straight line or a curved path. Transducers along such a line can have width such that the array has substantial area. Companies manufacturing such arrays have capabilities of varying the number of elements, size and shape of elements, and surface forms. An example is Acoustic Imaging Technologies, Inc. Some arrays involve transducers that are distributed over two dimensions to form a matrix surface. Representative two dimensional arrays are disclosed by U.S. Pat. No. 6,291,927 B1 Percin et al. (September 2001) and 2001/0044995A1 Tezuka (November 2001). These are both limited to planar surfaces. They also involve construction of the array surface as an assembly. Electrical connections are then attached by various methods to form a working system. 2001/0044995A1 Tezuka (November 2001) provides for layers that contain rows of connecting wires, and the layers are assembled with backing material prior to attaching a shell of piezoelectric material that is then cut into individual transducers.
Surface acoustic wave (SAW) devices use a form of ultrasonic waves for signal processing of electrical signals. One such use is for implementing delay lines where acoustic wave devices provide respective delays for signal channels. Appropriately delayed signals are combined electrically, using analog or digital techniques.
Bulk acoustic wave (BAW) devices use another form of ultrasonic waves for carrying out Fourier transform operations on time based signals.
The process of receiving wave signals with an array of transducers, summing signals from that array to form a signal and and distributing and transmitting that signal from a second array of transducers is a common practice in communications. This would be generally referred to as a relay process. A single channel communicates from the input system to the output system. Such a process is not a one-to-one mapping, rather a signal at a sample point on an input array becomes part of each signal at all sample points on the output array.
Common television maps signals from a focal plane in a television camera to pixels on a television receiver. In these systems wavefronts arriving at the camera lens are focused at the camera focal plane such that there is not a phase relationship between signals at the various points on the camera focal plane. Such signals are not spatially coherent. The focal plane sensor operates as a power detector such that the mapping does not carry phase information forward. Signals that are the result of power detection eliminate phase information and are, thus, non-coherent signals.
While there is an extensive background for the present invention, no prior art has effectively captured the generality of optical devices and their counterparts in combination with the generality of electronic signal processing.